What would social difference (racial/ethnic) in the U.S be like as an outsider?
I am going to assume that you are asking about social difference as perceived by someone who is from a non-dominant group in the United States.
If so, I would say that social differences based on race or ethnicity can often be the defining fact of life for people in the non-dominant group. The reason I say this is because all Americans live in a society in which white people are seen as the "norm."
Any person of color living in the United States knows that this is a white country. This is not surprising given that at least 75% of the population is white. Whether or not it is surprising, it is a real fact of life. Those who are not white live their lives surrounded by images of white people as the representatives of what is truly American. Just as one example, most characters in most TV shows are white. This sort of thing makes it so that people of color are constantly reminded of their outsider status.
This is a phenomenon that is much commented upon. For example, we have Ellison's Invisible Man and DuBois's idea of double consciousness among African-Americans.